RADIOACTIVE FALLOUTBY W. F. LIBBY

UNITED STATES ATOMIC ENERGY COMMISSION

Communicated May 29, 1958

I. Introduction.-The radioactivity produced by the detonation of nuclearweapons has been extensively studied and reported upon.'-'8 From this workwe have learned about the amount of radioactive fallout which occurs and themechanisms for its dissemination in a broad and general way. A few of thesegeneral points are as follows:

1. The stratosphere plays an extremely important role for the fallout frommegaton-yield weapons, and the troposphere is the medium which disseminatesthe fallout from kiloton detonations; thus, speaking broadly, stratospheric debrisis from megaton-yield detonations and the tropospheric fallout is from those oflower yield. It is not that the yield of the detonation is determinative but ratherthat the altitude to which the fireball rises before its average density is equalizedwith that of the surrounding air determines the fallout rates. The megaton-yieldfireballs are so enormous that they stabilize at levels only above the tropopause-the imaginary boundary layer dividing the upper part of the atmosphere, thestratosphere, from the lower part, the troposphere-while the kiloton-yield fireballsstabilize below the tropopause. The tropopause normally occurs at something like40,000-50,000 feet altitude, although it depends on season and location. In otherwords, low-yield bombs fired in the stratosphere would be expected to give thesame slow fallout rates as high-yield weapons do when fired in the troposphere -oron the surface, if attention is focused on the part of the fallout which does not comedown locally to form the oval-shaped pattern pointed in the downwind direction.

2. The stratospheric debris descends very slowly, unless, of course, it is solarge as to fall in the first few hours. This paper is concerned only with the world-wide fallout-that is, the fallout which does not occur in the first few hours-andexcludes the local fallout, which constitutes the famous elliptical pattern that isso hazardous because of its radiation intensity but which, in test operations, iscarefully restricted to test areas. It is worth mentioning in passing that the localfallout may be the principal hazard in the case of nuclear war. Most seriousattention should be paid to it in civilian defense programs.The world-wide fallout from the stratosphere is literally world-wide in that the

rate of descent of the tiny particles produced by the detonations is so small thatsomething like 10 years or somewhat less probably is the average time they spendbefore descending to the ground, corresponding to an average annual rate of about10 per cent of the amount in the stratosphere at any given time. It is not clear asto just how they do finally descend. It seems probable that general mixing of thestratospheric air with the tropospheric air, which occurs as the tropopause shiftswith season and as it is brought about by the jet streams, constitutes the mainmechanism and that the descent of the stratospheric fallout is never mainly due togravity, but rather that the bulk mixing of stratospheric air with tropospheric airbrings the radioactive fallout particles down from the stratosphere into the tro-posphere, where tropospheric weather finally takes over. The mechanism makesthe percentage fallout rate the same for all particles too small to fall of their own

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weight-and the same as would be expected for gases, provided that some means ofrapidly removing the gases from the troposphere exists; hence the reverse processof troposphere to stratosphere transfer does not confuse the issue.

3. World-wide radioactive fallout in the troposphere is restricted to the generallatitude of the detonations, for the reason that the residence time in the troposphereis about 30 days. 19-22 The lifetime of fine particles in the troposphere appears tobe determined by the cleansing action of the water droplets in the clouds. Forthose particulates which are below 1 jA in diameter, Greenfield2' calculates that themean residence time of a 1-pu particle in a typical cloud of water droplets of 20 1Ain diameter may vary between 50 and 300 hours, but that a particle of 0.04-pdiameter will last only 30-60 hours and that a particle of 0.01-JA diameter willlast only 15-20 hours. The theory calculates the diffusion due to Brownian motionand says that it is just this motion induced by the collisions with the air moleculesthat makes possible the contact between the fallout particles and the cloud drops.Since this theory is based on first principles, with the single assumption that thefallout particle sticks to the water droplet on impact-an assumption so plausibleas to be almost beyond doubt-it is no surprise to learn experimentally that theGreenfield theory appears to be correct.There is essentially no world-wide fallout in the absence of rainfall-i.e., in

desert regions-except for a little that sticks to tree leaves, blades of grass, andgeneral surfaces, by the same type of mechanism as that Greenfield describes inthe case of clouds. Thus we see that it is the moisture in the troposphere whichassures the short lifetime of the world-wide fallout particles and that, when thestratospheric air, which contains essentially no moisture and therefore has nocleansing mechanism, descends into the troposphere, the tropospheric moisture pro-ceeds to clean it up. On this model, we see that, for submicron fallout particles,weather phenomena are controlling and that the bombs which have insufficientenergy to push their fireballs over the troposphere will have their world-wide falloutbrought down in raindrops in a matter of about a month, in extreme contrast withthe stratospheric material, which apparently stays aloft for something like 10years, on the average. The contrast between these two lifetimes means that theconcentration of radioactive fallout in the stratospheric air in terms of equal densitiesof air is always much higher than in tropospheric air. This has been experimentallyobserved to be true.23 In fact, the stratospheric content is about 100-fold higherthan that of the troposphere, corresponding to the much longer stratosphericresidence time. Later in this paper, new data on the fallout content of the strato-sphere are given.

It is inherent in the Greenfield mechanism that the total world-wide fallout willbe proportional to rainfall if other factors are not allowed to vary. Thus we findthat the Mediterranean basin10 affords a good example of the truth of this principle.Other regions are northeastern United States, southeastern United States, northwest-ern United States, and southwestern United States.24 It is now well established thatdesert areas have very little fallout.

4. After falling to the ground in the form of rain or being picked up on thesurface of the leaves of grass or trees by the same type of Brownian-motion accretionmechanism that causes cloud-drop pickup, the -radioactive fallout may enter thebiosphere by normal biological processes. Radioactive strontium-90 and radio-

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active cesium-137 are the twoprincipal isotopes which have this facility, are producedin high yield by the fission reaction, and are of long enough lifetimes to be dis-seminated world-wide, particularly by the stratospheric mechanism-about 28years half-life for each. Strontium-90 is produced at a level equivalent to about 1millicurie of strontium-90 per square mile of the earth's surface for every 2 megatonsof fission energy, and radiocesium is produced at about 50 per cent higher yield.Of the two isotopes, strontium-90, because of its chemical similarity to calcium,collects in human bone, where it is held for years and where its radiations mightthen cause deleterious effects on the health of the individual, such as leukemia orbone cancer. It is interesting that strontium-90 constitutes a relatively lessimportant genetic hazard because of the short range of its radioactive radiationand the fact that it is not held in the reproductive organs. Radiocesium stays in thehuman body only 6 or 8 months, on the average, because it has no permanentstructure like the bone for which it has a natural affinity. As a result, the amountof radiation occurring from internally ingested radiocesium is much less and mostlikely is subject to palliative measures calculated to reduce its time in the body.Strontium-90 taken into the bone, however, appears to be stored for many years,the exact time not being known very well.2 Radiostrontium is taken into thebody because of its similarity to calcium, but there is a definite difference in chem-ical behavior which causes animal organisms to prefer calcium. Thus the radio-strontium content of newly deposited bone calcium is less than that for foodcalcium. In many countries the principal source of calcium is milk products;hence the fact that cow's milk has only one-seventh the strontium in it per gram ofcalcium that the cow's food has and that milk taken into the human body similarlydeposits calcium in the bones with only half the strontium-90 content of the milkitself- means that human beings naturally have a lower strontium-90 to calciumratio for new bone than for the food source by something like a factor of 15 fordairy' products. On the other hand, vegetation containing strontium-90 alsodeposits its strontium relatively inefficiently, with a factor of something like 4less strontium in the bone from these sources than is carried in the vegetable fooditself-all relative to calcium. In some countries where calcium in the human dietcomes principally from vegetables, other sources of calcium contribute, some ofwhich contain essentially no strontium-90-namely, sea food. Because fallout isdiluted so quickly by the action of the waves in the ocean, the concentration of theradioactive strontium in the sea calcium is very much lower than it is in the soilof the land in which the grass and vegetable crops grow. This difference becomeseven larger when the effects of direct leaf and stem base pickup are considered.This perhaps accounts for the high values reported by Ogawa26 for rice in Japan.Hence, fish from the sea are naturally at the lowest level in radiostrontium andsea- food should be the lowest source of calcium among ordinary human foods.With all these factors taken together, the world populations assimilate calcium ata much lower radiostrontium content than is exhibited by land plants. Eckel-mann, Kulp, and Schulert'8 have given a detailed sample calculation recently,based on their extensive measurements on human bone, and Comar has given thegeneral principles for this type of calculation.27

5. The biological hazard from the radioactive fallout from weapons-testing isnot well known, and, like many biological problems, the determination of the

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hazard in any exact way seems to be almost impossibly difficult. Fortunately,however, it is possible to compare the radiation from radioactive fallout with theintensities of natural radiation to which we are always exposed. For example, itis clear that the present level of the radiostrontium in the bones of young children,which are, of course, closest to being in equilibrium with the fallout, since adultshave had their bones some time even before there was any radioactive fallout, isabout 2 milliroentgens per year as compared to an average natural dosage of 150--200 milliroentgens per year, about 1-2 per cent of the dosage from natural sourcesto the bones, depending upon location. Natural radioactivity present in the ground,building materials, and even our own bodies gives us an average total dose at sealevel of about 150 milliroentgens per year, and medical X-rays add something likeanother 150 milliroentgens. The radiocesium taken into the body and the penetrat-ing radiations from non-assimilable radioactive fallout contribute perhaps another3 or 4 per cent to the whole-body dosage. Thus the total dosage to freshly formedhuman bone is, at most, 5 per cent of the natural dosage. Furthermore, we do knowthat the variations in natural background dosages from place to place are enormousin magnitude as compared to the average value and, of course, as compared to thefallout dosage. For example, it has been found28 that exposure rates from externalradiation rise from a value of about 90 milliroentgens per year at sea level to some-thing like 150 milliroentgens per year at 5,000-6,000 feet altitude in the UnitedStates. These numbers are considerably larger than those expected on the basis ofearlier calculations and measurements,3 29-31 the increase apparently being due to thecosmic rays and their increase with altitude. In addition, the effects of radio-activity in the soil and in building materials made of stone or soil are considerable,amounting in some instances to 50 or 100 per cent of the average natural backgrounddose at sea level, and the magnitude of the medical exposures to X-rays approxi-mates, on the average, those due to all natural sources.32We see, therefore, that, whatever the extent of our ignorance of the biological

effects of radiation, we do know that these effects are not unexperienced by thehuman species, even from the genetic point of view, since it is clear now thatpersons living at high altitudes on granitic rocks always have received extra radia-tion many times greater than is contained in the radioactive fallout from the testingof nuclear weapons and that even those living on certain sedimentary rocks at -ealevel always have received about ten to twenty times the present fallout dose.Of course, this does not mean that any of the effects from radioactive fallout are

in any way negligible, and it does not mean that certain numbers of people will notbe injured by radioactive fallout radiations, even though these numbers be verysmall relative to the total population of the world. However, the problem isbounded, and common sense and good judgment can be brought to bear on theextent of the biological hazards, even though they are not now known exactly andprobably will not be well understood for many years. Researches to increase thisunderstanding are being done, in the United States and United Kingdom and othercountries. Information on radioactive fallout and all its aspects, both physicaland biological, is collected and collated by the United Nations Scientific Committeeon the Effects of Atomic Radiation, which is drafting its first report at the presenttime.

6. From our study of radioactive fallout from testing, we have learned much of

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0

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Sr" FALLOUTMONTHLY RAIN WATER COLLECTIONS

HASL

( EACH CURVE OFFSET THREE DIVISIONS)

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FIG. 2.

value about the circulation of the atmosphere of the world, and we have muchmore to learn as the study continues, particularly in the stratosphere by balloonand aircraft sampling techniques being carried out principally in the United Statesat the present time. As we undertake the problem of locating the fallout in theoceans, we undoubtedly will learn much of interest to oceanographers about thecirculation of the water in the seas.

7. From our understanding of radioactive fallout from tests, we are the betterable to devise methods of civilian defense against fallout in the case of nuclear war,

and widespread popular interest in the potential possible hazards from radioactivefallout from nuclear tests has led to a considerable understanding on the part ofthe general public of these strange phenomena. From this debate and study maycome the protection for millions if nuclear war should occur.

Understanding of the nature of the mechanism by which radioactive falloutis disseminated has led to the reduction of the offsite fallout from testing. Weknow now that bombs placed upon the ground produce relatively more local fall-

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out and therefore less world-wide fallout. It seems likely that firing on the surface

of the sea has a similar, though probably- considerably less marked, effect.

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STRONTIUM 90 IN ANIMAL BONE

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II. Recent Data and Their Implications.-Figures 1, 2, 3, 4, and 5 and Tables1 and 2, which are up-to-date versions of earlier publications, give the most recentresults for the fallout observed for rainfall collections, for the strontium-90 contentof milk (fresh and dry), for human bone, and for animal bone. It is particularly

TABLE 1*SR90 IN FALLOUT AT MONITORING SITES OUTSIDE CONTINENTAL UNITED STATES

(High-walled Stainless-Steel-Pot Collections)me Sr98/mi2

Precipitation Observed Calculated(Inches) me Sr90/mi2 from Theory

Bangkok, Thailand (140 N.):March, 1957 1.95 0.05 0.085April, 1957 5.85 0.13 0.25WaY,) 1957 1.56 0.037 0.068June, 1957 9.36 0.016 0.41July, 1957 6.63 0.022 0.29August, 1957 11.70 0.039 0.051

Nagasaki, Japan(360 N.):August, 1956 17.43 0.34 0.76September, 1956 16.1 0.17 0.71October, 1956 3.59 0.20 0.16November, 1956 1.44 0.08 0.063December, 1956 1.37 0.22 0.069January, 1957 3.94 1.01 0.172February, 1957 3.28 0.17 0.143March, 1957 1.40 0.38 0.061April, 1957 11.27 1.98 2.4May, 1957 6.44 0.72 1.3June, 1957 10.18 0.27 2.1July, 1957 28.67 1.07 6.0August, 1957 11.35 0.457 2.4September, 1957 14.74 0.260 3.1* Program administered and monitored by Health and Safety Laboratory of New York Operations Office,

USAEC.

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TABLE 1-Continuedme Sr90/mi2

Precipitation Observed Calculated(Inches) me Sr9O/mi2 from Theory

Hiroshima, Japan (350 N.):August, 1956 11.93 0.50 0.52September, 1956 9.83 ... ...

October, 1956 3.51 0.27 0.15November, 1956 1.64 0.11 0.072December, 1956 0.23 0.06 0.010January, 1957 2.15 0.29 0.097February, 1957 2.26 0.53 0.098March, 1957 1.29 0.23 0.056April, 1957 11.00 1.12 2.3May, 1957 6.44 0.567 1.35June, 1957 10.22 0.493 2.2July, 1957 21.10 0.817 4.4August, 1957 4.48 0.047 - 0.94September, 1957 10.92 0.277 2.3

Rio de Janeiro, Brazil (230 S.):September, 1956 1.95 0.12 0.085October,1956 3.12 0.21 0.135November, 1956 3.51 0.06 0.155December, 1956 3.51 0 02 0.155January, 1957 2.73 0.04 0.12February, 1957 5.07 0.05 0.22

Salisbury, South Rhodesia (200 S.):November, 1956 7.41 0.18 0.32December, 1956 7.80 0.12 0.34January,1957 5.85 0.11 0.26February, 1957 8.97 0.08 0.04March, 1957 5.46 0.05 0.24April, 1957 1.17 0.04 0.05

Kikuyu, Kenya (00):January, 1957 9.75 0.14 0.43February, 1957 2.34 0.26 0.10March,1957 3.12 0.03 0.13April, 1957 7.02 0.03 0.31May, 1957 14.82 0.138 0.64June, 1957 1.56 0.187 0.068July, 1957 0.08 0.148 0.0035August, 1957 0 20 0.020 0.0087September, 1957 2.34 0.038 0.10

Dakar, French West Africa (140 N.):August, 1957 5.2 0.532 0.23September, 1957 10.44 0.244 0.45

Durban, Union ofAouth Africa (300 S.):June, 1957 0.39 0.080 0.017July, 1957 0.39 0.012 0.017August, 1957 0.78 0.096 0.034September, 1957 4.64 0.230 0.21

Pretoria, Union of South Africa (300 S.):July, 1957 4.29 0.061 0.187August, 1957 1.56 0.074 0.068

Vienna, Austria (470 N.):June, 1957 0.78 0.45 0.29July, 1957 5.07 1.95 1.85August, 1957 2.73 0.79 1.0

Klagenfurt, Austria (470 N.):August, 1957 3.51 1.17 1.3

PRECIP- OBSERVED PRECIP- OBSERVEDITA- MC SR90/MI2 ITA- MC SRE/MI2 MC SRE/MI2TION AEC Weather TION UNIVERSITY CALCULATED

(INCHES) Lab. Station (INCHES) OF HAWAII FROM THEORYOahu, Hawaii (200 N.):June,1957 0.32 0.72 ... 0.83 0.58 0.036July, 1957 2.10 1.36 0.477 1.62 0.42 0.071August, 1957 1.57 0.303 0.156 3.09 0.306 0.134September, 1957 1.54 0.274 0.188 0.62 0.159 0.027

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TABLE 1-ContinuedSR" IN FALLOUT AT OTHER UNITED STATES MONITORING SITES

(High-walled Stainless-Steel-Pot Collections)

Lemont, Illinois (440 N.):December, 1956January, 1957February, 1957March, 1957April, 1957May, 1957June, 1957July, 1957August, 1957September, 1957

PRECIPITATION OBSERVED(INCHES) MC SR90/MI2

1.26 0.142.06 0.301.77 0.271.98 0.476.09 1.153.21 0.275.94 0.488.98 1.575.36 0.691.08 0.12

MC SRU0/MI2CALCULATEDFROM THEORY

With WithoutRussian Russian

Component Component

0.510.380.720.802.51.51.953.62.10.41

0.0530.0880.0760.0851.30.681.01.91.10.21

Birmingham, Alabama (330 N.):April, 1957May, 1957June, 1957July, 1957August, 1957September, 1957

MC SRO0/MICALCULATEDFROM THEORY

PRECIPITATION OBSERVED With U.S. Without U.S.(INCHEs) MC SR/OMI2 Component Component

5.412.967.702.624.199.59

0.830.390.950.801.100.42

1.10.621.60.550.872.0

0.230.130.330.110.370.41

Salt Lake City, Utah (380 N.):December, 1956January, 1957February, 1957March, 1957April, 1957May, 1957June, 1957July, 1957

West Los Angeles, California (340 N.):December, 1956January, 1957February, 1957March, 1957April, 1957May, 1957June, 1957July, 1957

With WithoutRussian Russian

Component Component

1.671.370.722.183.243.371.470.31

0.310.80.832.392.300.811.610.94

0.660.540.290.871.41.50.660.13

Precipitation Observedt(Inches) me Sr"0/mi2

0.493.881.940.951.330.270.060.03

0.150.990.760.090.840.240.120.92

0.0710.0580.0310.0930.650.700.310.06

me Sr9"/mi2Calculated

from Theory

0.020.160.080.0410.280.0560.0120.006

South Miami, Florida (260 N.):April, 1957May, 1957June, 1957July, 1957August, 1957September, 1957t Some local fallout from Nevada.

With U.S. Without U.S.Component Component

5.0410.115.828.513.66.27

0.530.500.561.510.750.52

1.072.101.281.72.81.3

0.220.440.270.350.580.27

interesting to note that the data continue to show thepreviously and that little new in principle has appeared.

principal features noted

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Figure 6 shows preliminary data from the AECprogram on the stratospheric content of strontium-

_no< " 90. The data are preliminary for the reason that:>s00 the air-filter efficiencies are unknown at the present,

: 5° ° ° <tX2 although estimated to be something like 25 per cent.The samples are taken by pumping stratospheric air

,:,X through filters, which are then analyzed. Even.~-< though an enormous scatter is present for reasons of

°.°. time and experiment, it is clear that there ise no large variation in the stratospheric content of: strontium-90 between the latitude of 300 S. and the

Northern Hemisphere. Since most of the megaton-s~°c yield explosions have occurred in the northern lati-sc:s tudes, though the Pacific Testing Grounds are only

110 north of the equator, it appears that this evi-dence argues for rapid north and south mixing inthe stratosphere. As we shall see later, other evi-

C dence in the dissemination of non-radioactive carbon*000Sdioxidederived from the combustion of fossil fuels33-37

and in the dissemination of bomb-derived radioac->U tive carbon-14 seems to confirm this.38-40 It is in-

c9Ada, teresting to note also that the actual content of the055 stratosphere is not in disagreement with the esti-

mates given earlier,5 6 15 although the value of the fil-= ter efficiencies remains to be settled, and it is es-

<)0 .t ,: timated at the efficiency of about 25 per cent ona:On evidence assuming homogeneity of the particle size.5oo t Experiments are now under way to settle the point.

In the model previously advanced,' 6, 15 it is pro-posed that material introduced into the stratosphere

,ia, 0 ° is mixed immediately horizontally to a uniform con-.O:-t centration and has a residence time of 10 years.

o Further, it is assumed that the latitudinal spread ofs -3 tropospheric bomb clouds is only 100, with a sharp>>:< step function rather than a normal error-curve dis-

tribution. The bomb debris is arbitrarily assigned^ to the stratosphere except for 1 per cent tropospheric~ tin the case of megaton yields. Local fallout is as-

sumed to be 80 per cent for land surface shots, 20per cent for surface-water shots, and zero pet centfor air shots. All kilo-yield shots are assigned to thetroposphere. On these very simple bases we are

: then, from classified data about the magnitudes and: 'U nature of the explosions, able to estimate the to-

C3 B ala tal fallout for any place on earth if the deposition.ce from the troposphere is assumed to be proportional

*¢v> to the rain content at a given location. Figure 7gives such a theoretical latitudinal fallout profile

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FIG. 6.

for world-wide fallout as of December, 1957, neglecting rainfall variation, and Figure8 is the corresponding world map. Figure 9 gives the corresponding timewise varia-tions in the nothern latitudes and compares them with the rainfall fallout curves forMilford Haven in England.4' Figure 10 gives a similar comparison for Chicago andPittsburgh. Curves for other latitudes are given in Figures 11 and 12. Figure 13gives the estimated stratospheric reservoir and the expected composition in strontium-89 versus time. If a further assumption is made, namely, that the proportion of thefallout in a given location is given by the ratio of the rainfall to the world-wideaverage, 0.77 meters,42 it is possible to compare the detailed fallout observed bythe pot-collection programs in various localities with the theoretical predicted val-ues, and these are given in Table 1.On the basis of these comparisons and in the absence of conclusive evidence as to

the age of radioactive fallout, it appears that the simple theory outlined explainsthe known information within the experimental error. It may develop when more

reliable data are available on the age of fallout through the use of the short-lived,12.8-day half-life barium-140 fission product that a mechanism by which a sort ofconcentrated leaking from the stratosphere occurs at a latitude of about 40° more

may be proved or disproved. At the present time the observed extreme concentra-tion may be explained as being due to coincidence of the tropospheric fallout fromthe United States and Russian tests. If this theory be correct, the barium-140content in periods of high fallout will show that the fallout is young. It is to behoped that these data will be forthcoming soon.

Machta,13' 43 and Stewart, Osmond, Crooks, and Fisher4' have stated thatmeteorological considerations and likely stratospheric wind patterns, together with

STRATOSPHERIC Sr" CONTENT

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evidence that the Sr89/Sr90 ratio of the fallout shows the fallout to be old, have ledthem to the conclusion that the heavier fallout observed in the 40-50 N. latitudeband is stratospheric and not tropospheric in origin, as proposed here. Theissue still seems to be unsettled, since the radiochemical difficulties of the deter-

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WORLD FALLOUT MAPAl ktND 0F 1957

9 26

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1948 1949 1950 1951 1952 1953 1954FIG. 9.

1955 1956 1957 1958

mination of the Sr89/Sr90 ratio are large and may well have introduced sizable errorsinto some of the reported values for this number and since it apparently is possibleto account reasonably well for the observed fallout distribution on the presentuniform stratospheric fallout theory as shown in the present paper. The criticaldifference between the two theories is in the matter of the age of the fallout. Better

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PHYSICS: LIBBY PROC. N. A. S.

10 0 U. S. AND RUSSIAN

10

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1945

1948 1949 1950 1951b 1952 1953 1954 1955 1956 19S7 1956

FIG. 10.

30

PREDICTED WO FALLOUT CURVETOTAL FALLOUT

350 N-45* N

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1948 1949 1950 1951 19S2 1953 195 1955 1956 1957 1956

FIG. 11.

and more significant results probably will be available soon, using the Ba140/Sr9Oratio, which for both radiochemical and lifetime reasons is more suitable thanSr89/Sr90. Moreover, Ba'40 has a half-life of 12.8 days, which is more appropriateto distinguishing between an expected fallout age of perhaps 30 days, on the one

hand, and of about 1-2 years, on the other, than is the Sr89 half-life of 51 days.The radiochemical procedure for Ba'40 is very similar to that for Sr9O, and both are

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VOL. 44, 1958 PHYSICS: LIBBY

PREDICTED Sr9 FALLOUT CURVESTOTAL FALLOUT

45. SS° N

TOTAL 45.- N /(RUSSIAN + STRATOSPNERE) /

RUSSIAN BOMBS

STRATOSPNERIC_i

1947 1948 1949 1950 1951 192 1953 1954 1955 1956

FIG. 12.

1952 1952 l9U195 15s 1956 1957

FIG. 13.

more sensitive and reliable than the Sr89 procedure, which is particularly susceptibleto errors from radioactive impurities such as other fission products which may have

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PHYSICS: LIBBY

been imperfectly separated. Both Ba'40 and Sr9° are measured by short-livedradioactive daughters of characteristic half-life, which can be repeatedly removedand measured, since a new supply is grown into equilibrium each time a separationhas been made.The importance of settling this point is obviously considerable for both meteorol-

ogy and geophysics and certainly for the understanding of the mechanism ofradioactive fallout. Perhaps the Ba140 data will show the truth to lie somewherebetween the two mechanisms.

Rafter39 and Rafter and Fergusson28 have shown that carbon-14 increases insurface air at Makara in New Zealand and in New Zealand woods and oceancarbonate, as shown in Figure 14. This additional carbon-14 is due to bomb-generated neutrons which react with air nitrogen to produce it. They find about2A1 per cent increase per year.

BOMB C14 EFFECT21[

2 -TEXAS TREE RIIWAS

!I :: {RING (WILLIAMS OF HUMBLE OIL & RtEhMIWN CO.)

~~~~~~iWIS _9515| IN /YR.

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19S 1956 1957

al SUR^FACE CO, AT MAARA

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FIG. 14.

Williams,40 of the Humble Oil and Refining Company, finds 3.0 L 0.5 per centper year in Texas tree rings (Fig. 14), and de Vries44 in Holland and Miinnich*in Heidelberg, Germany, both report increases. The carbon-14 increase in theflesh of the land snail, Helix pomatia, amounted to 4.3 per cent between November,1953, and June, 1957, in Holland, while an increase of about 10 per cent during1955 and 1956 occurred in Heidelberg in various biosphere samples.At a rate of 2.5 neutrons per 200 Mev of energy release, 1 megaton would generate

3.2 X 1026 carbon-14 atoms. The best estimate, keeping in mind that a substantialamount falls back as calcium carbonate, would be that about 10" carbon-14 atoms'have been introduced into thq tnqsphere, mostly into the stratosphere. The

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PHYSICS: LIBB 817

estimate of 2.5 neutrons per 200 Mev energy released is higher than an earlierestimate based on an assumed 15 per cent escape efficiency,46 the later value beingbased on firmer information. It also attempts to weigh fusion and fission as theyhave actually occurred.About 9.4 X 1027 carbon-14 atoms are normally present in the stratosphere

because of cosmic-ray production.47 This figure assumes 22 per cent of the at-mosphere to be in the stratosphere. Therefore, with world-wide stratosphericcirculation, the rise in the stratosphere should be about 100 per cent, as was foundin a few measurements made on samples collected in October, 1956. Furthermeasurements are in progress.

1952 1953 1954FIG. 15.

1955 1 1956 1957

In the troposphere in the 3 years since the 1954 Castle test at the 10 per cent peryear figure used for fallout, about 3 X 1027 carbon-14 atoms should have descended,or about 1 X 1027 carbon-14 atoms per year. The average carbon-14 inventory inthe troposphere is 3.3 X 1028 without including the ocean or biosphere; hence theobserved carbon-14 rise might be as high as 3 per cent per year, as appears to havebeen observed.

If mixing with the biosphere and top ocean above the thermocline occurredimmediately, according to Arnold and Anderson,37 who gave 0.2 gm/cm2 in the top100 meters of the ocean, the total tropospheric reservoir would be 7.5 X 1028,giving an expected rate of increase due to the bombs of 1.3 per cent per year, which

800

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TRITIUM IN RAIN & SNOW

NMH- -Al.-4 OmKAI-4~IVY CASTLE U.S.S.R. REDWING

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0.~~~~~~*0 0 %

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817Voi.. 44, 1958

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PHYSICS: LIBBY

is in fair agreement with the observations if we assume that the mixing with theocean and the biosphere, particularly the former, is not quite instantaneous.The main points are that the ratio of the Northern to the Southern Hemisphere

effect here is not enormous and fits fairly well with the notion that stratosphericgases have a residence time not too different from that of the ultrafine world-widefallout particles.

In addition, Fergusson33 has recently found in studying fossil CO2 and its effecton reducing the carbon-14 content of the biosphere that the mean life of a CO2molecule before being absorbed from the tropospheric air into the oceans and bio-spheres is perhaps 2 years and that north-to-south mixing of the fossil CO2 occurs inless than 2 years. Consequently, it seems clear that the 10-year residence time forstratospheric gases before descent into the troposphere seems to fit data for carbon-14 from bombs as well as the strontium-90 and cesium-137 fallout data.The bomb tests to date have produced enough carbon-14 that, when it has come

to mixing equilibrium, it will have increased the amount naturally present in allliving matter by one-third of 1 per cent.The normal radiation dose from carbon-14 may be compared with the increase in

the dose from cosmic rays as the elevation increases. In these terms the normalcarbon-14 dose (1.5 mr/year) is equal to about a 100-foot increase in elevation.Therefore, the extra radiation dose from this product of nuclear tests is equivalentto an increase in altitude of a few inches.

In the years before equilibrium with the deep ocean is reached-about 500years-the level will rest temporarily at about a 3 per cent increase or the equivalentof a 3 foot altitude increase. This is after the first period of perhaps 10 or 20 yearsbefore dilution in the top layer of the ocean and with living and dead organic matteroccurs, when the increase will be about 20 per cent, or about 20-foot equivalentaltitude increase. Because the lifetime of radiocarbon is very long-8,000 yearson the average-the equilibrium situation is the more significant.

Figure 15 gives up-to-date data on the occurrence of tritium in rainwater in theChicago area.22 48,4 It is clear that, whereas strontium-90 and probably carbon-14 remain in the stratosphere for years, the tritium from high-yield thermonucleardetonations does not, but descends in a matter of 1 or 2 months. This is mostprobably due to the enormous mass of water carried into the stratosphere by thefireballs of detonations in the moist tropospheric air. The characteristic whitemushroom cloud is evidence of the formation of ice crystals in the cold stratosphericair, which, if large enough to be seen in this way, must certainly be large enough tofall into the troposphere, where they melt and join in the ordinary phenomena,i.e., fallout as rain or snow. Thus a large fractionation relative to fission productsand radioactive carbon dioxide occurs. Of course, there probably is some entrain-ment of fission products on the surfaces of the falling ice crystals by the Green-field Brownian-motion accretion mechanism. In fact, it is known that about 1per cent of megaton yield offsite fallout occurs in the early banded troposphericmanner. This may be due to this entrainment, and thus one would expect that thelatitudinal distributions of early tropospheric fallout of both fission products andtritium water from magaton-yield bombs fired in the troposphere8 should be identical.No satisfactory data are now available to check this point. In the calculationsin this paper the figure of 1 per cent for tropospheric contribution from megatonyields has been used.

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III. Conclusion.-The more recent data, particularly on bomb carbon-14,when taken together with the earlier data on bomb fission products and tritium.give us some confidence in our present understanding of the fallout mechanism.All these observations and considerations afford unprecedented opportunities forthe study of meteorology and geophysics, particularly in an international co-operative effort such as the International Geophysical Year.

1 World-wide Effects of Atomic Weapons, Project Sunshine (R-251-AEC) (amended), August 6,1953.

2 Merril Eisenbud and J. H. Harley, Science, 121, 677-80, 1955.3W. F; Libby, Science, 122, 57-58, 1955.The Hazards to Man of Nuclear and Allied Radiations (London: British Medical Research

Council, 1956).W. F. Libby, these PROCEEDINGS, 42, 365-90, 1956.W. F. Libby, ibid., pp. 945-56.Merril Eisenbud and J. H. Harley, Science, 124, 251-55, 1956.L. Machta, R. J. List, and L. F. Hubert, Science, 124, 474-77, 1956.E. A. Martell, The Chicago Sunshine Method (AECU-3262), May, 1956.

10 E. A. Martell, Project Sunshine Bulletin, 12 (AECU-3298) (Rev.), August 1, 1956.11 Y. Hiyama, Gakujutsu Geppo, 10, 1-17, 1957.12 Y. Hiyama, ibid., pp. 27-43.13Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy:

The Nature of Radioactive Fallout and Its Effect on Man, May 27-29, June 3-7, 1957 (Washington:Government Printing Office, 1957), Parts 1 and 2.

14 E. Dahl, Teknisk Ukeblad, July, 1957.16 W. F. Libby, these PROCEEDINGS, 43, 758-75, 1957.16 J. L. Kulp, W. R. Eckelmann, and A. R. Schulert, Science, 125, 219-25, 1957.17F. J. Bryant, A. C. Chamberlain, A. Morgan, and G. S. Spicer, Radiostrontium in Soil, Grass,

Milk, and Bone in the United Kingdom: 1956 Results, (A.E.R.E. HP/R 2353) (1957).18 J. L. Kulp, W. R. Eckelmann, and A. R. Schulert, Science, 127, 266-74, 1958.19 N. G. Stewart, R. N. Crooks, and E. M. R. Fisher, The Radiological Dose to Persons in the

U.K. Due to Debris from Nuclear Test Explosions Prior to January, 1956, (A.E.R.E. HP/R 2017)(1956).

20 0. Haxel and G. Schumann, Z. Physik, 142, 127-32, 1955.21 S. M. Greenfield, J. Meteorol., 14, 115-25, 1957.22 Haro von Buttlar, and W. F. Libby, J. Inorg. & Nuc. Chemn., 1, 75, 1955.23 N. G. Stewart, R. N. Crooks, and E. M. R. Fisher, The Radiological Dose to Persons in the

U.K. Due to Debris from Nuclear Test Explosions, (A.E.R.E. HP/R 1701) (1955).24 NYO-4889, "A Study of Fallout in Rainfall Collections from March through July, 1956,"

W. R. Collins and N. A. Hallden, April 30, 1957. NYO-4751, "Summary of Analytical Resultsfrom the HASL Strontium Program to June, 1956," J. H. Harley, E. P. Hardy, Jr., G. A. Welford,I. B. Whitney, M. Eisenbud, August 31, 1956. NYO4862, "Summary of Analytical Results fromthe HASL Strontium Program July through December 1956," J. H. Harley, E. P. Hardy, Jr.,I. B. Whitney, and M. Eisenbud.

20 W. F. and Margaret W. Neuman, Chemical Dynamics of Bone Mineral (Chicago: Universityof Chicago Press, 1958).

26 J. Ogawa, Bull Atomic Scientists, 14, 35-38, 1958.27 C. L. Comar, R. S. Russell, and R. H. Wasserman, Science, 126, 485,1957; C. L. Comar and

R. H. Wasserman, Proc. Intern. Conf. Radioisotopes and Scientific Research (in press); C. L.Comar and R. Scott Russell, Proc. Intern. Conf. Radioisotopes and Scientific Research (in press).

28 L. P. Solon, W. M. Lowder, A. V. Zila, H. D. LeVine, H. Blatz, and M. Eisenbud, Science, 127,1183, 1958.

29 P. R. J. Burch, Proc. Phys. Soc. London, A, 67, 421, 1954.30 H. V. Neher, Science, 125, 3257, 1957.31 These PROCEEDINGS, 1956.

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32B. P. Sonnenblick, J. Newark Beth Israel HoSp. (Newark, N.J.), 2, 81, 1957.3" G. J. Fergusson, Proc. Roy. Soc. London, A, 243, 561-74, 1958.34 H. E. Suess, Science, 122, 415, 1955." H. Craig, Tellus, 9, 1-17, 1957.36R. Revelle and H. E. Suess, Tellus, 9, 18-27, 1957.37 J. R. Arnold and E. C. Anderson, Tellu8, 9, 28-32, 1957.38 T. A. Rafter and G. J. Fergusson, Science, 126, 557, 1957.39 T. A. Rafter, New Zealand J. Sci. Technol., 37B, 20, 1955; T. A. Rafter, ibid., 38B, 871-83,

1957.40 M. Williams, private communication.41 N. G. Stewart, R. G. D. 0smond, R. N. Crooks, and E. M. R. Fisher, The World-wide Dep-

osition of Long-lived Fission Products from Nuclear Test Explosion (A.E.R.E. HP/R 2354) (1957).42K. Rankama and T. G. Sahama, Geochemistry (Chicago: University of Chicago Press, 1950).43L. Machta, "Discussion of Meteorological Factors and Fallout Distribution," paper presented

at AAS Meeting, December, 1957.44H. de Vries, Science (in press).4 K. 0. Munnich, private communication.46W. F. Libby, Science, 123, 657, 1956.47 W. F. Libby, Radiocarbon Dating (2d id., Chicago: University of Chicago Press, 1955).488. Kaufman and W. F. Libby, Phys. Rev., 93, 1337, 1954.4" F. Begemann and W. F. Libby, Geochim et Cosmochim. Aca, 12, 277-96, 1957.

THE HUMAN ELECTROHYSTEROGRAM: WAVE FORMS ANDIMPLICATIONS*

BY S. D. LARKS, PH.D.

DEPARTMENT OF BIOPHYSICS, SCHOOL OF MEDICINE, UNIVERSITY OF CALIFORNIA AT LOS ANGELES

Communicated by H. W. Magoun, May 16, 1958

Notwithstanding Bode's observation' of the deflection of a galvanometric needleduring a uterine contraction, the development of bioelectric studies of the uterusin situ (electrohysterography) has proceeded slowly and sporadically. Studiesboth abroad" 3 4' 5 and in this country6' 7. have made contributions. With ourdemonstration of a uterine electrical complex bearing a one-to-one relationship tothe uterine contraction,9' 10, 11 the studies have been placed on a firmer basis. Inthis communication certain recent findings which indicate pacemaker function andpropagated waves will be presented and their implications discussed.

MATERIALS AND METHODS

In the course of these investigations 293 subjects have been studied in labor, allunder conditions of maximum comfort and quiet. Skin overlying the uterus isprepared by rubbing in ecg paste which is then washed off. Pairs of bipolar elec-trodes are placed on the uterus at 10-cm. spacing, or for unipolar recording theexploring electrode is over the uterus and the indifferent electrode on the thigh.Direct coupled recording systems are essential for the slow events of the uterinecontraction. One-centimeter German silver electrodes and 5-mm. solder electrodeshave been used. Details of the technique have been described.9

RESULTS

For a better understanding of these studies, one of the first successful recordingsof the human electrohysterogram is here reproduced (Fig. 1). Time and amplitude

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